"""Concrete, checkable translation of the cPanel account ceiling (Chapter 03).

Single source of truth for the account's literal resource limits. Other
chapters should import BUDGET rather than re-hardcoding these numbers.
validate_budget_config() is called from create_app() so an out-of-budget
deployment fails loudly at startup instead of silently degrading under load
— this is the concrete mechanism behind Chapter 03's "flag, don't silently
accept" requirement.
"""
from __future__ import annotations

from dataclasses import dataclass

from flask import Flask


@dataclass(frozen=True)
class AccountBudget:
    """The hosting account's literal ceiling (Chapter 03).

    iops/io_throughput are informational only here — Python config can't
    enforce them directly; they constrain how Ch06/07 batch reads/writes.
    """

    cpu_cores: int = 4
    max_entry_processes: int = 60
    memory_mb: int = 2048
    iops: int = 1024
    io_throughput_mb_s: int = 16
    max_total_processes: int = 150
    max_db_connections: int = 150

    # Derived engineering targets from Chapter 03's budget table.
    db_pool_size_min: int = 2
    db_pool_size_max: int = 5


BUDGET = AccountBudget()

_DISALLOWED_CACHE_BACKENDS = {"redis", "rediscache", "memcached", "memcachedcache"}


def validate_budget_config(app: Flask) -> None:
    """Raise at startup if config violates a Chapter 03 rule.

    Cheap checks only (string/int comparisons) since this runs on every
    process boot — Passenger may recycle processes frequently (factor 9).
    """
    # Validated as its own config key, not read out of
    # SQLALCHEMY_ENGINE_OPTIONS — that dict is empty for SQLite (its pool
    # classes reject pool_size/pool_recycle outright; see app/config.py's
    # _engine_options_for), so the *intended* setting must be checked
    # independently of whether the active engine actually consumes it.
    pool_size = app.config.get("DB_POOL_SIZE")
    if pool_size is None or not (BUDGET.db_pool_size_min <= pool_size <= BUDGET.db_pool_size_max):
        raise RuntimeError(
            f"DB_POOL_SIZE={pool_size} is outside the Chapter 03 budget "
            f"({BUDGET.db_pool_size_min}-{BUDGET.db_pool_size_max} per process; "
            f"{BUDGET.max_db_connections} total connections are shared across "
            f"up to {BUDGET.max_entry_processes} entry processes)."
        )

    cache_type = str(app.config.get("CACHE_TYPE", "")).lower()
    if any(name in cache_type for name in _DISALLOWED_CACHE_BACKENDS):
        raise RuntimeError(
            f"CACHE_TYPE={app.config.get('CACHE_TYPE')!r} assumes a backend "
            "(Redis/Memcached) Chapter 03 says not to assume is available. "
            "Use FileSystemCache or a dashboard_cache DB table (Ch06)."
        )

    batch_size = app.config.get("PARSE_BATCH_SIZE")
    if not batch_size or batch_size <= 0:
        raise RuntimeError(
            "PARSE_BATCH_SIZE must be a positive integer — unbounded/whole-file "
            "parsing per invocation violates the Chapter 03 memory budget."
        )

    max_upload_mb = app.config.get("UPLOAD_MAX_SIZE_MB")
    if not max_upload_mb or max_upload_mb <= 0:
        raise RuntimeError(
            "UPLOAD_MAX_SIZE_MB must be a positive integer to bound disk/IOPS per upload."
        )